1. Technical Field
The present application relates to a porous silica material used as an acoustic propagation medium, and an optical microphone.
2. Description of the Related Art
An optical microphone is a device for detecting a sound wave using light. Specifically, by taking in a sound wave into a transparent acousto-optic medium, the sound wave is converted to a compression wave propagating through the acousto-optic medium, and the compression wave is detected as temporal variations in the optical length using a vibrometer optical system such as a laser Doppler vibrometer (LDV), thus detecting the sound wave. Therefore, in order to realize an optical microphone having desirable characteristics, it is very important to develop an acousto-optic medium for efficiently converting a sound wave to variations in optical properties.
As such an acousto-optic propagation medium, Japanese Laid-Open Patent Publication No. 2009-85868 (herein after, referred to as Patent Document No. 1) discloses a porous silica material formed by a silica and an optical microphone using the same. Now, the structure and the operation of the optical microphone disclosed in Patent Document No. 1 will be described, and the function and the importance of an acousto-optic propagation medium in an optical microphone will be described.
FIG. 25 is a schematic diagram showing a configuration of a conventional optical microphone disclosed in Patent Document No. 1. A conventional microphone 101 includes a reception section 120, a detection section 121, and a conversion section 110.
The reception section 120 takes in a sound wave propagating through the environment around the reception section 120, and converts the sound wave to variations in optical properties. For this, the reception section 120 includes a base 103 having a depressed portion 103a, and a transparent support plate 107 supported so as to cover the opening of the depressed portion 103a. An acousto-optic propagation medium portion 102 made of a porous silica material is arranged in the space formed by the depressed portion 103a of a base 103 and the transparent support plate 107. Also, the depressed portion 103a includes an acoustic waveguide 106, one surface of which is defined by a top surface 102a of the acousto-optic propagation medium portion 102. The base 103 includes an opening 104 for allowing the sound wave to enter the acoustic waveguide 106.
The detection section 121 detects, using light, the variations in the optical properties which have occurred in the reception section 120. The detection section 121 is a laser Doppler vibrometer (abbreviated as LDV), and includes a head 108 and a calculation section 109.
A sound wave having propagated through the air propagates from the opening 104 into the acoustic waveguide 106 along a sound wave propagation direction 105. As the sound wave propagates through the acoustic waveguide 106, the sound wave enters the porous silica material of the acousto-optic propagation medium portion 102 through the top surface 102a of the acousto-optic propagation medium portion 102, and propagates through the acousto-optic propagation medium portion 102.
A laser beam 100 output from the head 108 toward the acousto-optic propagation medium portion 102 passes through the transparent support plate 107 and the acousto-optic propagation medium portion 102 to be reflected at a bottom surface 103c of the depressed portion 103a of the base 103. The reflected laser beam 100 passes again through the acousto-optic propagation medium portion 102 and exits from the acousto-optic propagation medium portion 102 to be received by the head 108. When a laser beam 100 passes through the acousto-optic propagation medium portion 102, the density and the refractive index of the porous silica material of the acousto-optic propagation medium portion 102 vary due to the propagation of the sound wave therethrough, and the laser beam 100 is modulated by these variations.
The laser beam 100 received by the head 108 is converted to an electric signal and is then output to the calculation section 109. The calculation section 109 processes the electric signal to output, to a conversion section 110, a modulated component contained in the laser beam 100 as a detection signal. The conversion section 110 converts the detection signal to a sound pressure to output a received signal. The conversion in the conversion section 110 is as follows.
The volume V of the porous silica material of the acousto-optic propagation medium portion expands/shrinks due to the sound pressure of the sound wave propagating through the inside of the acousto-optic propagation medium portion 102, thereby causing a volume change ΔV. Due to the volume change ΔV, the refractive index n of the porous silica material 102 changes by Δn. These relationships are represented by Expression (1).
                                          Δ            ⁢                                                  ⁢            V                    V                =                  -                                    Δ              ⁢                                                          ⁢              n                                      n              -              1                                                          (        1        )            
Since the sound wave is propagating through the inside of the porous silica material as an acoustic plane wave, the volume change ΔV is limited to displacement in the sound wave traveling direction. Therefore, Expression (2) holds true.
                                                                                          Δ                  ⁢                                                                          ⁢                  V                                V                            =                            ⁢                              -                                                      Δ                    ⁢                                                                                  ⁢                    l                                    l                                                                                                        =                            ⁢              S                                                          (        2        )            
Herein, l is the length of the porous silica material in the sound wave propagation direction, Δl is the displacement in the sound wave propagation direction of the porous silica material due to the sound wave propagation therethrough, and S denotes the distortion of the porous silica material in the sound wave propagation direction. The sound pressure inside the porous silica material can be represented by Expression (3) with the distortion S and the elastic constant E of the porous silica material.P=−S×E  (3)
The elastic constant E can be represented by Expression (4) using the density ρ and the sound speed C of the porous silica material.E=C2×ρ  (4)
From Expressions (1) to (4), the sound pressure P of the inside of the porous silica material can be represented by Expression (5) using the density ρ, the sound speed C and the refractive index n of the porous silica material, and the displacement output ΔL, which is an electric signal output from the head 108, or the velocity output v, which is the output of the calculation section 109.
                                                        P              =                            ⁢                                                -                                      C                    2                                                  ×                ρ                ×                                                      Δ                    ⁢                                                                                  ⁢                    n                                                        n                    -                    1                                                                                                                          =                            ⁢                                                -                                      C                    2                                                  ×                ρ                ×                                                                                                                                                ⁢                      n                                                              n                      -                      1                                                        ·                                                            Δ                      ⁢                                                                                          ⁢                      L                                        L                                                                                                                          =                            ⁢                                                -                                      C                    2                                                  ×                ρ                ×                                                      n                                          n                      -                      1                                                        ·                                      1                    L                                                  ⁢                                  ∫                                      v                    ⁢                                          ⅆ                      t                                                                                                                              (        5        )            
Therefore, using the velocity output v output from the calculation section 109, it is possible to measure the sound pressure P.
Non-Patent Document No. 1, Sumio Sakka, “Application of sol-gel method”, Agne Shofu Publishing Inc., Jul. 31, 1997 (P44-45) discloses a silica aerogel as a material having a particularly small modulus of elasticity among porous silica materials that are suitable for such an application. Non-Patent Document No. 1 states that the silica aerogel has a very small density of 0.03 to 0.3 g/cm3, and the silica aerogel is useful as an acousto-optic propagation medium for an optical microphone.
The structure of a porous silica material used in a conventional optical microphone, and a method for manufacturing the same will now be described. A low-density porous silica material, commonly referred to as “silica aerogel”, has many pores and 90% by volume thereof consists of voids. The skeleton is formed by spherical silica particles of about some nm to some tens of nm connected together.
Commonly, a porous silica material is produced by allowing a sol liquid including an alkoxide of silicon to undergo hydrolysis and polymerization reaction, thereby producing a wet gel, and then replacing the solution in the wet gel with a gas. When the solution is replaced with a gas, i.e., dried, the gel structure will be destroyed if the tensile stress based on the surface tension of the solution remaining in the pores is greater than the strength of the gel. In order to prevent this, supercritical drying is often used in the step of drying the wet gel. Also, a porous silica material with very little aging can be obtained by subjecting the gel to a hydrophobization process. Non-Patent Document No. 1 discloses, as a hydrophobization process, a method for replacing a hydroxyl group remaining after hydrolysis and polycondensation with a hydrophobic modified group.